1. Field of the Invention
The present invention relates to a method for forming a deposited film and an apparatus for forming a deposited film, specifically to a film formation method and film formation apparatus for forming a thin film semiconductor and a photovoltaic element by, for example, a plasma CVD method, and relates to a film formation method and a film formation apparatus for forming a semiconductor thin film of a photovoltaic element such as a solar cell, using amorphous silicon or an amorphous alloy. More specifically, the present invention relates to a film formation method and a film formation apparatus for forming an amorphous silicon-based deposited film with a good and uniform quality in a large area while maintaining a high rate of film formation.
2. Related Background Art
By using amorphous silicon produced by the plasma CVD method, a semiconductor device having a large area can be comparatively easily formed as compared with use of crystal silicon or polycrystal silicon. Therefore, many amorphous silicon films are used for semiconductor devices, each of which needs a large area, such as a solar cell, a photosensitive drum of a copying machine, an image sensor of a facsimile, and a thin-film transistor for a liquid crystal display.
Formation of an amorphous-silicon film is generally conducted by a plasma CVD method in which a raw material gas containing Si such as SiH4 or Si2H6 is decomposed with a high frequency discharge to change the gas into a plasma state and form a film on a substrate placed in the plasma.
When forming an amorphous-silicon film in a large area by the plasma CVD method, a high frequency of RF frequency (near 13.56 MHz) has been generally used conventionally. However, in CVD apparatuses which each have been developed for attempting to correspond to a large area in response to requests from devices having enlarged substrates, a problem of “non-uniformity of film quality” which was in tolerance level in each conventional apparatus corresponding to a small area has become larger in recent years.
Moreover, since from a viewpoint of productivity improvement acceleration of a film formation rate has been also requested in connection with enlargement of a substrate, several methods described below have been performed.
Typical methods are:
(1) a method of increasing a flow rate of a raw material gas such as SiH4 or Si2H6,
(2) a method of increasing the electric power of a high frequency discharge, and
(3) plasma CVD method using a VHF frequency (about 30 to 300 MHz), which attract attention as leading technology.
For example, it is reported in Amorphous Silicon Technology 1992 p. 15-26 (Materials Research Society Symposium Proceedings Volume 258) that, by making a discharge frequency into a VHF frequency from a 13.56 MHz RF, a film formation rate can be markedly increased, and the formation of a good deposited film can be attained at a high rate.
However, in large area film formation, although any one of the above-described methods is effective in a point of increasing a film formation rate, they are not effective in regard to the above-described uniformity of film quality, and in addition, have problems that they reduce the whole film quality and the like.